Communication Cite This: Chem. Mater. 2018, 30, 602−606
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From AgGaS2 to Li2ZnSiS4: Realizing Impressive High Laser Damage Threshold Together with Large Second-Harmonic Generation Response Guangmao Li,† Yu Chu,‡ and Zhongxiang Zhou*,† †
Department of Physics, Harbin Institute of Technology, Harbin 150001, China University of Chinese Academy of Sciences, Beijing 100049, China
‡
S Supporting Information *
= 84.6 × KDP, Eg = 2.22 eV),48 etc. From mentioned investigations, it is clear that (1) sulfides have relative smaller SHG responses whereas have wider band gaps than selenides; (2) the substitution with heavier elements in the same group may lead to the decrease of band gaps; (3) more than one kind of metal-centered polyhedral in the structure could lead to large SHG responses. For example, a series of lithium-induced sulfides and selenides reported by Aitken et al. have good properties such as Li2CdGeS4. Previous work32 shows that introduction of the d10 cations Zn2+ with small atomic radii can enlarge the SHG responses but has almost no effects on the band gap because almost no occupation in the bottom of conduction band (CB) and the top of the valence band (VB) exists. Besides, introduction of Si can help to improve the SHG responses in oxides as well as sulfides, such as Cs2B4SiO9,18 BGa2SiS6,48 Li2SiIn2S6,49 etc.; the Si atom is also small enough to sustain a large band gap. Guided by this design strategy of a cosubstitution method,51 taking AGS as the template, we chose the alkali metal Li to substitute transition metal Ag and replaced the SHG active units3,48−50,52 GaS4 with the ZnS4 and SiS4 units and successfully obtained one new promising IR NLO material Li2ZnSiS4. It shows excellent properties, including a large band gap of 3.9 eV (much larger than AGS, 2.67 eV), high LDT (10 × AGS) and large SHG response (Type-I phase-matching (PM), 1.1 × AGS) at 2.09 μm. Theoretical calculations show that the SHG coefficient d33 is 18.89 pm/V and both ZnS4 and SiS4 units produce the main contribution for the macroscopic SHG response. All the results indicate that Li2ZnSiS4 is a promising candidate in the IR frequency-conversion region. Li2ZnSiS4 was synthesized using the mixture of elemental Li, Zn, Si, and S as raw materials with the stoichiometric ratio at 900 °C, then cooling down to room temperature in 1 week. Airstable millimeter level single crystals were obtained by spontaneous crystallization, which indicates its good growth habit. Crystal photograph and powder X-ray diffraction (PXRD) spectrum are presented in Figure 1. The experimental result matches well with the theoretical one, which Li2ZnSiS4 crystallizes in Pna21 space group of orthorhombic system with unit cell parameters of a = 12.892(2) Å, b = 7.7739(12) Å, c = 6.1451(10) Å, and Z = 4. Crystal and refinement data, atomic
N
onlinear optical (NLO) materials, including borates, phosphates, chalcogenides, etc., are important materials in many highly technological industries and military science, so they have appealed to more and more researchers.1−19 Herein, infrared nonlinear optical (IR NLO) materials20−32 have attracted much attention in recent decades because of their spectacular applications, including space communications, the detection of hazardous materials for homeland security, etc. Commercially applied IR NLO materials, AgGaS2 (AGS),33−36 AgGaSe2,33 and ZnGeP2,34 exhibit large second-harmonic generation (SHG) responses but low laser damage threshold (LDT) or two-photon absorption (TPA) induced by small band gaps, which limit their applications. To obtain large band gaps and high LDTs within IR NLO materials, scientists have chosen the alkali metal Li to substitute the transition metal Ag and harvest a series of materials37−40 like LiGaS2, LiGaSe2, etc. This substitution leads to large improvement in their band gaps because alkali metals can avoid the d−d and f−f electron transition. However, the wide band gap is corresponding to a high LDT but inversely proportional to the large SHG coefficients in one material.41 For example, LiGaS2 has the larger band gap of about 4.15 eV than AgGaS2 and an evidently high LDT (11 times that of AgGaS2), but its SHG response decreases to half that of AgGaS2.38 Moreover, two lithium sulfides (Li2BaMS4, M = Ge, Sn)10 with large band gaps (>3.0 eV) have been reported by Wu et al, whereas their SHG responses turn to be ∼0.5 × AGS. This phenomenon tells us that the introduction of alkali metal Li is useful to improve the band gap. However, the problem to maintain the large SHG effect is still a challenge. According to the anion group theory,42 SHG active groups are the anion groups in the materials. As for metal chalcogenides, their SHG responses may be originated from the metal-centered Q-coordinated (Q = S, Se, Te) anion polyhedrons.28−32 Investigations tell that the MS4 units (M = Zn, Hg, Si, Ge, etc.)11,23,28−32 are practicable because they may have large distortions and make a positive effect on macroscopic SHG response. Many IR NLO crystals exhibit large SHG responses like ternary AgGaS2 (dij = 33.3 × KDP, Eg = 2.67 eV), AgGaSe2 (dij = 84.6 × KDP, Eg = 1.83 eV),33 BaGa4S7 (dij = 33.3 × KDP, Eg = 3.54 eV),43 BaGa4Se7 (dij = 52.8 × KDP, Eg = 2.64 eV),44 etc., and quaternary Li2Ga2GeS6 (dij = 41.0 × KDP, Eg = 3.65 eV),45 LiGaGe2Se6 (dij = 47.7 × KDP, Eg = 2.08 eV),46 Li2CdGeS4 (dij = 65.6 × KDP Eg = 3.15 eV),47 BaGa2SiS6 (dij = 33.3 × KDP, Eg = 3.75 eV),48 BaGa2SiSe6 (dij © 2018 American Chemical Society
Received: December 26, 2017 Revised: January 16, 2018 Published: January 17, 2018 602
DOI: 10.1021/acs.chemmater.7b05350 Chem. Mater. 2018, 30, 602−606
Communication
Chemistry of Materials
Figure 1. (a) Theoretical and experimental PXRD patterns for Li2ZnSiS4; (b) photograph of crystals picture for Li2ZnSiS4.
coordinates and isotropic displacement parameters, and selected bond distances are listed in Tables S1−S3 in the Supporting Information. In its asymmetric unit, there are two unique Li atoms, one Zn atom, one Si atom, and four S atoms. The Li atoms are four-coordinated with S atoms to form the LiS 4 tetrahedra, and the adjacent Li(1)S 4 and Li(2)S 4 tetrahedra connect with each other through corner S(1) atoms to form the Li2S7 dimers (as shown in Figure 2a). These
Figure 3. (a) Topological structure of Li2ZnSiS4 with the Li2S7 units regarded as the nodes; (b) topological structure of AgGaS2 with AgS4 units regarded as the nodes.
units to be 4c nodes, the structure that AgS4 forms also turns to be a honeycomb-like tunnel framework and all Ga atoms are located in the tunnels as shown in Figure 3b. This double substitution lead to the only site of Ag(1) into two, Li(1), Li(2) atoms and one Ga(1) site into tow Zn(1) and Si(1) sites, respectively, which further make the symmetric of the whole structure lower, e.g., from tetragonal to orthorhombic system. Many other compounds in Li2−MII−MIV−Q4 (Q = S, Se) type have been reported by Aitken. Herein, the MII site could be occupied with Zn, Cd, Mn, Fe, and the MIV site could be seated with Ge and Sn. Table S4 lists the space groups and crystal systems of these compounds, together with that of Li2ZnSiS4 and AgGaS2. These compounds have diamond-like (DL) structures, which mean that (1) all the metal elements have four-coordination with Q atoms to form different tetrahedra; (2) all the tetrahedra connect with each other only through corner-sharing; (3) the orientation of all the tetrahedra is almost parallel. The alignment of the tetrahedra in DL structures leads to the geometrical superposition of the microscopic SHG tensors of the units and result in large SHG responses.19 Moreover, this kind of structure provides a large treasury for synthesis and design of new materials.53 Li2ZnSiS4 just has a DL structure with ZnS4 and SiS4 tetrahedra aligning in the same direction, which indicates its good NLO properties. Important optical properties have been characterized and will be analyzed herein. The UV−vis-NIR diffuse reflection data are collected and converted into absorbance using the Kubelka− Munk function54 to estimate the band gap of Li2ZnSiS4. The band gap is about 3.90 eV, which is much larger than that of AgGaS2 (2.67 eV). Investigations have shown that large band gap may indicate a high LDT of one compound, which encourages us to measure the LDT of Li2ZnSiS4 with AgGaS2 as the reference using a 1064 nm Q-switch laser. The incidence energy increases from 0.02 mJ and stopped when a dark spot is discovered under an optical microscope. The measured samples are ground powders from hand-picked crystals for both compounds. The measured cutoff energy of Li2ZnSiS4 and AgGaS2 are 0.7 and 0.07 mJ respectively, which means that the LDT for Li2ZnSiS4 is about 10 times that of AgGaS2, and comparable with that of LiGaS2 (reported to be 11 times that of AgGaS2). The high LDT of Li2ZnSiS4 promises its promising application in the high-energy laser system. The infrared spectrum has also been measured and shows no obvious absorption in the wavelength range of 3−25 μm (Figure S2).
Figure 2. (a) Li2S7 dimer formed by S(1)-shared Li(1)S4 and Li(2)S4; (b and c) wave-like Li2S5 chains viewed in different directions; (d) Li2S4 framework in ab plane; (e) alternately arranged Zn and S(2) atoms; (f) alternately arranged Si and S(1) atoms; (g) whole structure of Li2ZnSiS4.
Li2S7 dimers connect through sharing edges (S(3) and S(4) atoms) to form wave-like chains along the c-axis (as shown in Figure 2b,c), and then these chains interconnect through sharing S(2) atoms to form the tunnel framework (as shown in Figure 2d). The Zn and Si atoms are all four-coordinated with S atoms; herein, the Zn atoms insert between the S(2) atoms, whereas the Si atoms insert between S(1) atoms, as shown in Figure 2e,f, respectively. To make the complex structure clearer and briefer, Li2S7 dimers are considered to be 6 connected (6c) nodes. As a result, shown in Figure 3a, the Li2S7 dimers form a topological honeycomb-like tunnel framework, Zn and Si atoms located in the tunnels, which turns to be very similar to the structure of AgGaS2. In the asymmetric structure of AgGaS2, there is only one crystallographical Ag atom. Considering AgS4 603
DOI: 10.1021/acs.chemmater.7b05350 Chem. Mater. 2018, 30, 602−606
Communication
Chemistry of Materials
Figure 4. (a) Comparison on SHG responses between Li2ZnSiS4 and AgGaS2 at 2.09 μm at different particle sizes; (b) UV−vis-NIR diffuse reflection spectrum and band gap (inserted) of Li2ZnSiS4.
Thermogravimetric (TG) curve of Li2ZnSiS4 is shown in Figure S3. From the TG curve, we could see that Li2ZnSiS4 is stable in nitrogen gas until ∼730 °C. The SHG responses of Li2ZnSiS4 were measured on six different particle sizes under a 2.09 μm laser, and AgGaS2 with the same sizes act as the reference. The results show that Li2ZnSiS4 is type-I phase-matched, and its SHG response is 1.1 times that of AgGaS2 at the largest particle size, as shown in Figure 4a. Moreover, the calculated NLO coefficients for Li2ZnSiS4 are 9.97, 9.01, and 18.89 pm/V for d15, d24, and d33, respectively. The largest d33 coefficient is larger than that of AgGaS2 (13.9 pm/V). Compared with commercially applied IR NLO materials and some other famous IR materials (Table 1), Table 1. Properties Comparison among Li2ZnSiS4, LiGaS2, AgGaS2, and Some Popular Materials Formula
Eg (eV)
dij (× AGS)
LDT (×AGS)
AgGaS27−9 LiGaS210−13 LiInS255 BaGa4S716a
2.64 4.15 3.57 3.54 3.90
1 0.4 0.6 1 1.1
1 11 2.5 3 10
Li2ZnSiS4
Figure 5. (a) Calculated SHG density in occupied state; (b) calculated SHG density in unoccupied state.
Li2ZnSiS4 exhibits the more comprehensive properties, especially achieving the good balance between large SHG response and high LDT. It should be noted that Li2ZnSiS4 has comparable SHG response with benchmark AgGaS2, which shows our premier strategy that choosing ZnS4 and SiS4 as the SHG active units to maintain the SHG response is feasible. Besides, the substitution of Ag with Li has also been proved an efficient way to enlarge the band gap. To deeply understand the relationship between the properties and structures, theoretical calculations based on first principle theory with CASTEP package were performed, including the theoretical nonlinear coefficients, SHG density (shown in Figure 5), band structure (BS) (shown in Figure S1a), and total/partial density of states (T/P DOS) (shown in Figure S1b). In the SHG density spectrum, combining the occupied and unoccupied states, it is clear that both of ZnS4 and SiS4 units produce the main contribution to the SHG response, which matches well with our designation. The calculated nonlinear coefficients for Li2ZnSiS4 are 9.97, 9.01, and 18.89 pm/V for d15, d24, and d33, which also demonstrates its good NLO property.
In conclusion, we have successfully designed and obtained a promising IR NLO material, Li2ZnSiS4, which achieves the good balance between large SHG response and high LDT. Moreover, high-quality millimeter level single crystals were also obtained, which indicates that this material has good growth habits. Theoretical analysis demonstrates that both ZnS4 and SiS4 units produce the main contribution to the large SHG response. This study demonstrates that Li2ZnSiS4 is a promising IR NLO candidate that may be used in the highenergy laser system.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b05350. Crystal data, structural information on some selected compounds, experimental details, theoretical calculated 604
DOI: 10.1021/acs.chemmater.7b05350 Chem. Mater. 2018, 30, 602−606
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Chemistry of Materials
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band structure and density of states, infrared spectrum, thermal gravity analysis (PDF)
AUTHOR INFORMATION
Corresponding Author
*Zhongxiang Zhou, E-mail:
[email protected]. ORCID
Zhongxiang Zhou: 0000-0003-1952-7147 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by the Fundamental Research Funds for the Central Universities (Grant No. HIT. MKSTISO. 2016 11).
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